Recently liquid crystal color display devices are widely used because of their power and space saving properties. Pixels for a conventional liquid crystal color display device arrangement are illustrated in FIG. 1. As is shown in FIG. 1, a pixel 2 is composed of a set of three, red, blue and green, independently positioned liquid crystal cells. For each pixel 2, the strength of transmitted light is altered by an electric field, with an additive color mixture process being performed to provide a multi-color display.
FIG. 2 is a conceptual cross-sectional view of one pixel for a conventional liquid crystal display device. Voltages applied to the red, green and blue cells are adjusted by a TFT (thin-film transistor) 9, and the orientation of a liquid crystal layer 7 under the cells is changed in order to control the transmission of light emitted by a backlight 13. When white light transmitted through the liquid crystal layer 7 passes through red, green and blue color filters 5, red, green and blue colors are displayed, and the color for each pixel can be varied. An aluminum plate 12 is provided to efficiently reflect the light from the white backlight 13, and a pattern is screen-printed on a diffusion plate 15 to efficiently and uniformly reflect the light from the backlight 13.
With this arrangement, a problem exists in relation to increases in resolution. When pixel 2 is to be activated, three liquid crystal cells must be independently driven, and accordingly, three TFTs are required. Further, the sizes of cells must be reduced in order to increase the resolution. However to efficiently drive the liquid crystals, the size of the transistors is not reduced proportionately to the size of the cells. Further in accordance with an increase in resolution, the opening ratio for a cell (the ratio of a portion whereat light is transmitted and color can be seen, to a non-opening portion such as a transistor and wiring) is small and precisely forming three different color filters in a tiny area will increase the manufacturing costs for a liquid crystal color display device.
To resolve these problems, a new display method has been proposed. According to this method, color filters are removed from a liquid crystal color display device; the entire liquid crystal picture frame is changed from a red picture to a green picture, and to a blue picture in a field-sequential manner; and light illuminated from the rear is varied from red, to green, to blue in synchronization with the picture changes. This method is employed, for example, for a compact liquid crystal color display device (e.g., KOPIN Corporation's cyber Display (trademark) 320) that uses red, green and blue light emitting diodes for illumination. However, since the light emitting diodes are point light sources, a great number of diodes are required for a liquid crystal color display device that has a large picture frame, and manufacturing costs are very high.
A low-cost, cold-cathode fluorescent lamp having a high luminous efficiency for a liquid crystal color display device has been proposed by Bright Institute and others, wherein three cold-cathode fluorescent lamps, for red, green and blue, are pulse-operated (article on Nippon Keizai Shinbun, Jun. 5th, 1997). The switching time for a color picture frame display of this liquid crystal color display device is reported as 15 to 20 ms. However. acolor switching time of 5.56 ms required to obtain 60 pictures per second, which is the same as that of a television picture frame.
According to the method for pulse-operating three colored cold-cathode fluorescent lamps, the dynamic characteristic for luminous efficiency of a phosphor, which is coated on the tube of a fluorescent lamp, is extremely important. When rise and decay characteristics of the fluorescence of the phosphors are inferior, emitted light can not be used efficiently. Furthermore, light infiltrates a picture that is being written, or the preceding color is mixed with the colors of the next picture, reducing color purity.
Since there are few phosphors that have excellent fluorescent transient characteristics, it is difficult to find a phosphor that satisfies the need not only for excellent dynamic properties while providing other desired properties. For example, a phosphor having excellent dynamic properties may react with mercury that is included in a filler gas and deteriorate, or the peak of the luminous efficiency spectrum is not sharp, so that color quality is not always adequate.
Therefore, it is one object of the present invention to provide a liquid crystal color display device incorporating cold-cathode fluorescent lamps that employ phosphors having a fast transient characteristics suitable for employment in the above described field-sequential manner.
It is another object of the present invention to provide a liquid crystal color display device of a field-sequential type that can employ a phosphor that reacts with a filler gas containing mercury and is easily deteriorated.
It is an additional object of the present invention to provide a liquid crystal color display device of field-sequenitial type that demonstrates improved color rendering properties.